Inkjet Printhead with Nozzle Chambers Each Holding Two Fluids

1. An inkjet printhead comprising: an array of droplet ejectors, each having a chamber with a nozzle; and, an actuator for ejecting drops of ink through the nozzle; such that during use, each of the chambers hold ink and a gas, the gas remaining in the chamber during activation and deactivation of the actuator; wherein, the actuators each have a part with a hydrophilic coating for contacting ink in preference to other parts of the actuator such that the gas and the ink are in direct contact with each other at an interface within the chamber, and at least part of the actuator is positioned at the interface between the ink and the gas.

2. An inkjet printhead as claimed in claim 1 wherein part of each of the actuators has a hydrophobic coating such that the gas contacts the hydrophobic coated part of the actuator in preference to other parts of the actuator.

3. An inkjet printhead as claimed in claim 1 wherein each of the actuators moves a surface within the chamber to eject drops of ink.

4. An inkjet printhead as claimed in claim 1 wherein each of the actuators comprises an electrothermal heater positioned for heating ink in said chamber to a temperature above the boiling point of the ink, causing a bubble to form in order to eject a drop of the ink from said nozzle.

5. An inkjet printhead as claimed in claim 1 wherein each of the actuators is adapted to use differential thermal expansion upon Joule heating to cause ejection of, or assist the ejection of ink from said nozzle.

6. An inkjet printhead as claimed in claim 1 wherein each of the actuators comprises a material with a very high coefficient of thermal expansion, such that thermal expansion of the actuator causes ejection of, or assists the ejection of, ink from said nozzle.

7. An inkjet printhead as claimed in claim 1 wherein each of the actuators is capable of directly firing the ink drops from said nozzle, without the assistance of an external field.

8. An inkjet printhead as claimed in claim 1 wherein each of the actuators is a differential expansion bend actuator capable of converting a high force, low travel mechanism to high travel, lower force mechanism.

9. An inkjet printhead as claimed in claim 1 wherein each of the actuators bends when energized.

10. An inkjet printhead as claimed in claim 1 wherein the chamber, the actuator and the nozzle in each of the drop generators are configured such that ink refills the chamber after drop ejection by the surface tension of the ink.

11. An inkjet printhead as claimed in claim 1 adapted to maintain the ink at a positive pressure wherein each of the drop ejectors has an ink inlet, such that during use, ink in said chamber is restricted from flowing out of the ink inlet during drop ejection by the positive ink pressure.

12. An inkjet printhead as claimed in claim 1 wherein each of the drop ejectors has an ink inlet and each of the chambers has an ink inlet an ink pushing surface, the ink inlet positioned close to the ink pushing surface, such that during drop ejection, ink in said chamber is restricted from flowing out of the ink inlet by the ink pushing surface.

13. An inkjet printhead as claimed in claim 1 further adapted to energize said actuator in rapid succession for clearing dried or partially dried ink in said nozzle.

14. An inkjet printhead as claimed in claim 1 further adapted to provide an enhanced drive to said actuator for clearing dried or partially dried ink in said nozzle.

15. An inkjet printhead as claimed in claim 1 wherein the apparatus is formed on and through a substrate, the nozzle is formed on a front surface of the substrate such that ink flows through the substrate, and ink drops are ejected from the front surface.